Plating apparatus and method for rotary engine housings

ABSTRACT

An apparatus and method are disclosed for simultaneously highly uniformly electroplating the working surfaces of a plurality of rotary engine housings. Rotary engine housings are vertically stacked in a box around an insoluble hollow perforated conforming anode. The box is substantially closed except for congruent top and bottom openings that are concentric with and slightly larger than the conforming anode. During plating, the box is completely submerged in an electroplating solution. The box openings allow electroplating solution to enter the box and pass into the anode interior through the anode perforations. Electroplating solution is withdrawn from the anode interior and recirculated back to the tank.

United States Patent 1 1 Klingenmaier Dec. 30, 1975 [75] Inventor: )ttoJ. Klingenmaier, Warren,

Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: July 22, 1974 [21] Appl. No.: 490,688

[52] US. Cl. 204/26; 204/237; 204/272; 204/297 W [51] Int. Cl. C25D7/04; C25D 17/06 [58] Field of Search. 204/272, 26, DIG. 7, 237-239,204/240, 232, 212, 241, 259, 260, 275, 297 W [56] References CitedUNITED STATES PATENTS 2,487,399 11/1949 Thurber 204/288 3,514,389 5/1970Stephan et al. 204/237 3,640,799 2/1972 Stephan et a1. 3,840,440 10/1974Durin 204/25 Primary ExaminerT. M. Tufariello Attorney, Agent, orFirm-Robert J. Wallace [57] 1 ABSTRACT An apparatus and method aredisclosed for simulta neously highly uniformly electroplating theworking surfaces of a plurality of rotary engine housings. Rotary enginehousings are vertically stacked in a box around an insoluble hollowperforated conforming anode. The box is substantially closed except forcongruent top and bottom openings that are concentric with and slightlylarger than the conforming anode. During plating, the box is completelysubmerged in an electroplating solution. The box openings allowelectroplating solution to enter the box and pass into the anodeinteriorthrough the anode perforations. Electroplating solution iswithdrawn from the anode interior and recirculated back to the tank.

4 Claims, 2 Drawing Figures BACKGROUND OF THE INVENTION This inventionrelates to a method and apparatus for simultaneously electroplatingextremely uniform wearresistant coatings onto the radial workingsurfaces inside a plurality of i'otary engine housings.

More specifically, this invention involves electrodepositing extremelyuniform wear-resistant coatings onto the epitrochoidal working surfaceinside a rotary engine housing. This surface forms a radial wall of anepitrochoidal chamber within which a generally triangular rotoreccentrically rotates.

It is normal and accepted practice to electroplate a chromium layer ontothe epitrochoidal working surface of the rotary engine housing, toreduce working surface wear. Typically, the chromium layer is about0.003 0.006 inch thick. Accordingly, the housing chamber is normallymachined oversize to accommodate this thickness. A chromium layerthicker than needed is then plated onto the working surface, andmachined back to the desired thickness. An excess chromium thickness ofabout 0.005 0.006 inch is generally deposited in the process. Inmachining the chromium layer to the desired thickness, the finalepitrochoidal dimensions are obtained. The chromium plated workingsurface is ground smooth and relatively flat between axial faces on therotary engine housing, with sharp comers at those faces. After grinding,the chromium deposit may be anodically etched to provide a more oilretentive working surface.

The chromium can be electrodeposited onto the epitrochoidal workingsurface by stacking two or more bushings around a conforming anode. Onetechnique for doing so is described in U.S. Pat. No. 3,514,389 Stephanet al.

Excess chromium is deposited and then machined back to the desiredthickness because the chromium plating process inherently tends toproduce a nonuniform deposit. A significant proportion, about 60 80percent, of the cathode current in the process is spent generatinghydrogen gas. Increased current density, to accelerate the plating rate,increases gas evolution. The gas which is evolved produces a higherelectrical resistance nearer the top of an electroplating bath than thebottom. This results in faster deposition of chromium nearer the bottomof the tank than the top, and corresponding thickness variations inelectrodeposits. It is not unusual to obtain, as the result of gasevolution alone, at least 5 percent reduction in thickness for every 3inches in cathode height in the tank. Such conditions make itexceedingly difficult to chromium plate even one rotary engine workingsurface when the plate thickness is about 0.002 0.006 inch without aplus or minus 20 percent variation from the average mean thickness. Itis most practical to plate a plurality of housings simultaneously, andto stack them vertically. The greater the number of housings in thestack, the larger is the variation in chromium thickness from top tobottom of the stack. Greater solution agitation in the tank can helpreduce thickness variation and minimum chromium thickness needed for thestack. However, even with such aids the usual method of preparing rotaryengine housings is still to plate an excess thickness of chromium, andthen grind it back to size.

I have found a way to simultaneously chromium plate a plurality ofvertically stacked rotary engine housings very uniformly. Significantlylesser chromium layer thicknesses can now be used. This conserves bothchromium and energy. In many instances no excess chromium need bedeposited at all. The chromium layer can be deposited with suchuniformity that it is plated exactly to finish thickness. Moreover, inthese latter instances, the edges of the working surface are sharpenough so that no machining of the chromium coating is required at all.

OBJECTS AND SUMMARY OF THE INVENTION It is; therefore, an object of thisinvention to provide a method and apparatus for electrodepositing awearresistant coating of more uniform thickness onto the epitrochoidalworking surface of a rotary engine housanother object of this inventionis to provide a method and apparatus for electrodepositing a chromiumlayer onto the epitrochoidal working surface of rotary engine housingsthat substantially reduces, if not eliminates, finish machining of thechromium layer.

A further object of this invention is to conserve chromium and energy inthe chromium plating of epitrochoidal working surfaces of rotary enginehousings.

The objects of this invention are obtained with a method and apparatusin which a plurality of rotary engine housings are vertically stackeddirectly on each other around a conforming anode within a substantiallyclosed nonconductive container. The container is completely submerged inan electroplating solution in an electroplating tank. The epitrochoidalworking surfaces of the housings are in vertical alignment, and metalplates placed at the top and bottom of the stack. The metal plates haveopenings therein congruent with the aligned epitrochoidal workingsurfaces of the housings. Top and bottom walls of the container areadjacent the metal plates. These walls have epitrochoidal openingstherein smaller than and concentric with the epitrochoidal workingsurfaces of the aligned housing stack. The conforming anode is insolubleand uniformly perforated opposite the housings. Electroplating solutionin the tank enters the closed container through the openings in thecontainer top and bottom walls, passes between the anode and thehousings, and into the anode interior through the anode perforations.Means are provided to continuously withdraw electroplating solution fromthe anode interior and recirculate it back into the electroplating tank.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantagesof the invention will become more apparent from the followingdescription of preferred embodiments thereof and from the drawing, inwhich:

FIG. 1 is a sectional view along the line 1-1 of FIG. 2, and

FIG. 2 is a top plan view with parts broken away showing an apparatuscontemplated by this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference is now made to thedrawing which shows an electroplating tank 10 filled with a chromiumelectroplating solution at a temperature of about 60 C. The plating tankis of the usual type, as is the chromium electroplating solution. Onesolution that can be used is a water solution containing 230 grams perliter Cr O ric acid of about 88 92:1 is preferred over the usual 100:1ratio to obtain faster deposition rates.

An assembly is suspended within the tank for supporting the workpiecesto be plated. The assembly includes two generally U-shaped frame members12 and a cross member 14, secured to a lower horizontal portion 12 ofthe frame members 12. A first, or lower, flat plastic plate 16, about 14inches square, rests on lower horizontal portions 12 of frame members12. Plastic plate 16v is about '12 inch thick and has a generallycentral epitrochoidal opening 18 therein.

Four rotary engine housings 20 are stacked in vertical register with theupper axial surfaces of each housing abutting the lower axial surface ofthe housing above it. The epitrochoidal working surfaces are thusvertically aligned to form a closed vertical passageway 22 ofepitrochoidal transverse section. Any passageways (not shown) throughthe housings that intersect the epitrochoidal working surfaces arepreferably plugged in the usual manner for plating. The rotary enginehousings can be of cast aluminum, cast iron or the like. Theepitrochoidal working surface of each housing 20 forms the radial wallof an epitrochoidal chamber having a major radial dimension of about 9%inches, which dictates a minor radial dimension of about 7 inches. It iscontemplated that this invention canbe used with rotary engine housingsof any size, but especially for those with epitrochoidal chamber majorradial dimensions of about 9 to 11% inches.

The stack of housings is sandwiched between and in direct contact withan upper metal plate 24 and a lower metal plate 26, which are preferablyboth of steel and about '/4 inch thick. Both plates 24 and 26 haveepitrochoidal openings 28 and 30 in them congruent with passageway 22.The aligned housing stack and associated metal plates 24 and 26 areplaced on the-square flat plastic plate 16, concentric with theepitrochoidal opening 18 therein. A second, upper, flat and squareplastic plate 32 rests on top of the upper conductive plate 24. Upperplastic plate 32 is of the same thickness as lower plastic plate 16 butis only 12 inches square. It has a generally central epitrochoidalopening 34 therein identical to opening 18 in lower plastic plate 16.The upper plastic plate 32 is positioned so that its opening 34 iscongruent with opening 18, and therefore also concentric with passageway22. Epitrochoidal openings 18 and 34 have a major dimension of about 7.5inches and a minor dimension of about 5 inches. Plastic dowel pins 36and 36 can be used to maintain alignment and concentricity of thevarious parts. An open ended, plastic tubular member 38, having a squarecross section surrounds the stack of conductive plates and housings. Thelower end of tubular member 38 rests on the upper surface 40 of thelower plastic plate 16. The axis of tubular member 38 is perpendicularto the major surfaces of the plastic plates 16 and 32. Plate 16substantially closes the lower end of tubular member 38 and plate 32substantially closes the upper end. Conversely, the lower end of tubularmember 38 encloses a portion of surface 40 and plate 16. Tubular member38 has walls about 1% inch thick and fractionally more than 12 incheswide on their inner surface, to closely receive upper plastic plate 32.Plastic lower and upper plates 16 and 32 thus cooperate with thefoursided tubular member 38 to form a substantially closed box-likecontainer surrounding the stackof rotary engine housings. The plasticplates 16 and 32 and tubular element 36 can be of different thicknesses,so long as they are rigid and durable in the plating solution andcooperable to form a substantially closed chamber.

A tubular insoluble conforming anode 42 is concentrically positionedwithin passageway 22, so that its outer surface 44 is parallel thealigned housing working surfaces forming passageway 22. Tubular anode 42has a constant transverse section along its length. The transversesection is generally epitrochoidal, with a major dimension of about 6.5inches and a minor dimension of about 4 inches. The composition of theanode forms no part of this invention, and any of the usual insolubleanode compositions for chromium plating canbe used. For example, it canbe made of lead, titanium coated with lead, titanium coated with leadoxide, or copper coated with successive layers of iron and a lead-tinalloy containing 93 percent, by weight, lead.

The lower end of anode 42 is opposite lower metal plate 26 and is sealedby an insulated or nonconductive imperforate plate 45. Tubular anode 42extends upwardly through passageway 22 and the opening 34 in upper.plastic member 32. The upper end of anode 42 is above the level 46 ofthe electroplating solution in tank 10. Anode 42 has a generally uniformwall thickness of about 0.125 0.250 inch.

The portion of the anode above passageway 22 in contact with theelectroplating solution has a nonconductive coating 48. The portion ofanode 42 in passageway 22, directly opposite the metal plates and theworking surfaces of the housings, has a plurality of holes 50 radiallyarrayed throughout its length that communicate the anode interior withpassageway 22. The holes 50 are approximately '74; V4 inch in diameterand are positioned about 1 inch apart radially around the periphery ofthe anode and alternately spaced axially in rows about k inch apart.Both sides of the anode at its minor transverse dimension, or waistportion, are shielded with a plastic strip 52 and 52' about 1 /4 inchwide along the entire length of the passageway 22. Strip 52 serves as ashield and is only wide enough to maintain a generally constant ratio ofanode to cathode area radially around the anode. The minor dimension, orwaist, portions of the housing working surfaces are thus prevented fromplating at a faster rate than the balance of the housing workingsurfaces.

Two plastic pipes 54 and 54' are suspended within the interior of anode42 .from a manifold pipe 56 that communicates with conduit 58. Conduit58 communicates with the inlet side of pump 60, through a connectionat62. The outlet side of pump 60 is connected to conduit 64, whichcommunicates with the bottom of tank 10. In this way, electrolyte can becontinuously withdrawn from the anode interior during plating and pumpedback into the bottom of tank 10. As electrolyte is withdrawn from theanode interior it is continually replenished by electrolyte flowing intopassageway 22 through the bottom and top openings 18 and 34 in plasticmembers 16 and 32, respectively. Optimum pumping rates are in the rangeof about 12 24 liters per minute. The electrolyte flows through theopenings 18 and 34 and passes between the surfaces being plated and theouter periphery of the anode whereupon it can pass into the anodeinterior through holes 50 in the anode. Gases generated at the anode arecontinuously directly swept into the anode interior, to maintain a moreuniform anode to cathode resistance throughout the height of the housingstack. Also, it is preferred to agitate the electroplating solution inthe tank with suitable means (not shown) to maintain solution uniformitythroughout the tank.

A source of negative potential (not shown) is connected to a verticalstrap member 66, that is bolted to each of the rotary engine housingsand extends up out of the electroplating solution. It is preferred tomaintain a current density of about 39 amperes per square decimeter onthe housings. The strap 66 is preferably coated (not shown) in all areascontacting the electrolyte and not contacting the rotary enginehousings. The positive side of the potential source is connected to theanode to complete the plating circuit. The nonconductive plates 16 and32, tubular member 38 and dowel pins 36 and 36' can be of anonconductive material, or coated with a nonconductive material. Theycan be of plastic such as a polyniethyl methacrylate material. They canalso be of any ceramic or plastic coated metal that is nonconductive anddurable.

The spacing between anode 42 and the housing working surfaces is about1% to 2 inches for rotary engine housings having epitrochoidal chamberswith radial major dimensions of about 9 1 1.5 inches. The gap betweenthe anode and the openings 34 and 18 of the top and bottom plasticmembers 32 and 16 is about 0.5 inch. In general, the inner edge ofplastic plates 16 and 32 defining epitrochoidal openings 18 and 34should overlap the openings in their contiguous metal plates. Theoverlap should be at-least about one-half of and preferably abouttwo-thirds of the radial distance between the openings in the metalplates and the anode. Thus, for anode to plate spacing of about 1% 2inches, the plastic plate to anode radial spacing should be about f. toinch.

In using my apparatus, metal plate 26 is first placed on lower plasticplate 16. The rotary engine housings prepared for plating in the normaland accepted manner are then stacked on lower metal plate 26. Uppermetal plate 24 is then placed on top of the housing stack and plasticplate 32, in turn, placed on top of upper metal plate 24. The housingsand metal plates are aligned and concentrically positioned with theplastic plates. Plastic dowel pins 36 and 36 are then placed throughthem to insure that they stay uniformly positioned. Cathode conductorstrap 66 is bolted to the rotary engine housings. Box-like member 38 isthen placed around the stacked housings and plates. The assembly canthen be lowered into an electroplating tank, with the lower plasticplate 16 spaced at least an inch or two above the bottom of the tank andthe upper plastic plate 32 spaced at least an inch or two below thelevel of the electroplating solution in the tank.

Anode 42 can be permanently attached to cross member 14, and thehousings and plates stacked around it. On the other hand it can beremovable, and inserted in passageway 22 after the housings and plateshave been stacked. It might even be attached to a support above the tankand lowered into the assembly after the assembly is submerged in theelectroplating solution.

Further, the upper end of anode 42 can be closed with an end wall (notshown), with plastic pipes 54 passing through it. In such instance,anode 42 need not extend up above the level of upper plastic plate 32.

The foregoing describes a plating assembly that is portable and usefulin any plating tank with little tank modification. On the other hand,one can permanently install portions of such an assembly in anelectroplating tank, should it be desired. For example, the lowerplastic member 16 and the surrounding box-like enclosure 38 could bepermanently attached to a support on the bottom of the tank. Also, anode42 can be closed at its upper end, the anode mounted permanently inplace, and electroplating solution pumped out of the anode interiorthrough the lower end of the anode.

It should also be noted that this same assembly can be used to uniformlyanodically etch the chromium layer after it is deposited. In fact,although not normally preferred, anodic etching can be done in the sametank and solution by merely reversing polarity of the potential source.

I claim:

1. An apparatus for uniformly electrodepositing a wear-resistant coatingonto epitrochoidal radial working surfaces of a plurality of rotaryengine housings simultaneously, said apparatus comprising:

a nonconductive substantially closed box-like enclosure for containing avertical stack of abutting and registered rotary engine housings havingaligned epitrochoidal inner working surfaces forming a passageway havingan epitrochoidal cross section,

a flat metal plate on the top of said housing stack,

a flat metal plate on the bottom of said housing stack,

each of said flat metal plates having a major surface in contact withsaid stack and having an epitrochoidal opening therein congruent withsaid aligned inner surfaces,

top and bottom walls on said enclosure adjacent said metal plates witheach wall having an epitrochoidal opening therein concentric with andsmaller than the epitrochoidal cross section of said passageway,

an insoluble anode having a generally epitrochoidal transverse outerperiphery and an interior space, said anode coaxially extendingthroughout said passageway parallel to and radially spaced inwardly fromsaid aligned inner surfaces,

said anode being radially closer to said concentric epitrochoidalopenings in said enclosure top and bottom walls than to said alignedinner surfaces,

said anode having a plurality of radially arrayed openings throughoutsaid passageway that communicate said anode outer periphery with saidanode interior space,

means for preventing said solution from entering said anode interiorspace except through said anode openings in said passageway,

means for supporting said enclosure in a tank of electroplating solutionwith said enclosure completely immersed in said solution, and

means for continuously withdrawing said solution from said anodeinterior space and recirculating it to said tank duringelectrodeposition of said wearresistant coating onto said aligned innersurfaces.

2. An apparatus for uniformly electrodepositing a chromium coating ontoepitrochoidal radial working surfaces of a plurality of rotary enginehousings simultaneously, said apparatus comprising:

a nonconductive closed box-like enclosure for containing a verticalstack of abutting and registered rotary engine housings having alignedepitrochoidal inner working surfaces forming a passageway having anepitrochoidal cross section with a major dimension of about 1 12 inches,7

flat metal plates about 0.1 0.2 inch thick contacting the top and bottomof said housing stack, with each plate having an epitrochoidal openingtherein congruent with said aligned inner surfaces, flat top and bottomwalls on the interior of said enclosure adjacent said flat metal plateswith each -wall havingan epitrochoidal opening therein concentric withand having a major dimension of about 1 1.5 inch smaller than saidpassageway major dimension,

an insoluble anode having a generally epitrochoidal transverse outerperiphery and an interior space,

1 said anode coaxially extending throughout said passageway parallel toand radially spaced inwardly from said aligned inner surfaces,

said anode transverse outer periphery having a major dimension about 1.52.0 inches smaller than said passageway major dimension,

said major dimension of said concentric epitrochoidal openings in saidenclosure top and bottom walls being larger than said anode majordimension by about of the difference between said anode and saidpassageg major dimensions,

a tank having a bottom wall and containing a solution -forelectrodeposition of chromium,

means for supporting said enclosure in said tank completely immersed insaid solution and spaced above said tankbottom wall wherein saidsolution can enter said passageway through said enclosure top and bottomopenings, 'saidanode having a plurality of openings radially arrayedthroughout said passageway by which said solution can enter said anodeinterior space from said passageway, "means for preventing said solutionfrom entering said I anode interior space except through said anodeopenings in said passageway, and means for continuously withdrawing saidsolution from said anode interior space and recirculating it to saidtank during electrodeposition of said chromium coating onto said alignedinner surfaces of 'said housings.

3. An apparatus for uniformly electrodepositing a chromium coating ontoepitrochoidal radial working surfaces of a plurality of rotary enginehousings simultaneously, said apparatus comprising:

a frame for supporting workpieces to be electroplated in a tankcontaining chromium electroplating solution,

a horizontal first flat nonconductive plate on said frame,

a vertically oriented tubular nonconductive member with open upper andlower ends, the lower end of which, is sealed on and encloses an uppersurface portion of said first nonconductive plate, 1 a first horizontalflat metal plate on said enclosed upper surface portion within saidtubular member for contact with the bottom of a vertical stack ofabutting and registered rotary engine housings within said tubularmember, said housings having aligned epitrochoidal inner workingsurfaces forming a vertical passageway of epitrochoidal horizontal crosssection,

a second horizontal flat metal plate within said tubular member forcontact with the top of said stack of rotary engine housings,

a second horizontal flat nonconductive plate on top of said second metalplate, and substantially closing the upper end of said tubular member,

said first and second flat metal plates each having an epitrochoidalopening therein congruent with said aligned inner surfaces of saidrotary engine housin s,

said first and second flat nonconductive plates each having anepitrochoidal opening therein concentric with and smaller than saidmetal plate openings through which said electroplating solution in saidtank can enter top and bottom ends of said passageway, v

vertical tubular insoluble anode having an epitrochoidal horizontalouter periphery concentric with and smaller than said nonconductiveplate openings coaxially disposed in said housing passageway anduniformly radially spaced inwardly from said aligned inner surfaces, theradial spacing between said anode and said passageway being about two tothree times greater than the radial spacing from said nonconductiveplates,

said anode extending upwardly along the entire length of said passagewayand through said second plate opening to a level above them,

a nonconductive coating covering all anode portions outside saidpassageway and anode waist portions within said passageway, and

said anode having a closed bottom end and a plurality of uniformlyradially arrayed openings therein along its length through whichelectroplating solution from said passageway can pass within saidtubular anode and be recirculated to said tank during electroplating.

4. A method of uniformly electrodepositing a wearresistant coating ontoepitrochoidal working surfaces of a plurality of rotary engine housingssimultaneously, said method comprising the steps of:

vertically stacking a plurality of rotary engine housings havingepitrochoidal inner working surfaces in alignment to form a passagewayhaving an epitro-v choidal cross section,

stacking metal plates at the top and bottom of the housing stack withthe metal plates each having an epitrochoidal opening therein congruentwith said passageway,

substantially enclosing said stack of housings and metal plates in anonconductive container having top and bottom epitrochoidal openingstherein adjacent the stack that are concentric with and smaller than theepitrochoidal cross section of said passageway,

coaxially positioning a tubular insoluble anode having a hollow interiorand a generally epitrochoidal cross section within said passageway, withsaid anode paralleling the entire length of said passageway and spacedradially inwardly therefrom about two or three times its radial spacingfrom said container openings, said anode having radially arrayedopenings throughout said passageway communicating said passageway withthe anode interior,

completely immersing said container and said anode in an electroplatingbath so that electroplating solution can enter said container throughsaid top and bottom openings,

preventing electroplating solution from entering the anode interiorexcept through said radially arrayed openings,

applying a negative potential to said housing stack and a positivepotential to said anode, and

' tank.

1. AN APPARATUS FOR UNIFORMLY ELECTRODEPOSITING A WEARRESISTANT COATINGONTO EPITROCHOIDAL RADIAL WORKING SURFACES OF A PLURALITY OF ROTARYENGINE HOUSINGS SIMULTANEOUSLY, SAID APPARATUS COMPRISING: ANONCONDUCTIVE SUBSTANTIALLY CLOSED BOX-LIKE ENCLOSURE FOR CONTAINING AVERTICAL STACK OF ABUTTING AND REGISTERED ROTARY ENGINE HOUSINGS HAVINGALIGNED EPITROCHOIDAL INNER WORKING SURFACES FORMING A PASSAGEWAY HAVINGAN EPITROCHOIDAL CROSS SECTION, A FLAT METAL PLATE ON THE TOP OF SAIDHOUSING STACK, A FLAT METAL PLATE ON THE BOTTOM OF SAID HOUSING STACK,EACH OF SAID FLAT METAL PLATES HAVING A MAJOR SURFACE IN CONTACT WITHSAID STACK AND HAVING AN EPITROCHOIDAL OPENING THEREIN CONGRUENT WITHSAID ALIGNED INNER SURFACES, TOP AND BOTTOM WALLS ON SAID ENCLOSUREADJACENT SAID METAL PLATES WITH EACH WALL HAVING AN EPITROCHOIDALOPENING THEREIN CONCENTRIC WITH AND SMALLER THAN THE EPITROCHOIDAL CROSSSECTION OF SAID PASSAGEWAY, AN INSOLUBLE ANODE HAVING A GENERALLYEPITROCHOIDAL TRANSVERSE OUTER PERIPHERY AND AN INTERIOR SPACE, SAIDANODE COAXIALLY EXTENDING THROUGHOUT SAID PASSAGEWAY PARALLEL TO SAIDRADIALLY SPACED INWARDLY FROM SAID ALIGNED INNER SURFACES, SAID ANODEBEING RADIALLY CLOSER TO SAID CONCENTRIC EPITROCHOIDAL OPENINGS IN SAIDENCLOSURE TOP AND BOTTOM WALLS THAN TO SAID ALIGNED INNER SURFACES, SAIDANODE HAVING A PLURALITY OF RADIALLY ARRAYED OPENINGS THROUGHOUT SAIDPASSAGEWAY THAT COMUNICATE SAID ANODE OUTER PERIPHERY WITH SAID ANODEINTERIOR SPACE, MEANS FOR PREVENTING SAID SOLUTION FROM ENTERING SAIDANODE INTERIOR SPACE EXCEPT THROUGH SAID ANODE OPENINGS IN SAID PASSAGEWAY, MEANS FOR SUPPORTING SAID ENCLOSURE IN A TANK OF ELECTROPLATINGSOLUTION WITH SAID ENCLOSURE COMPLETLY IMMERSED IN SAID SOLUTION, ANDMEANS FOR CONTINUOUSLY WITHDRAWING SAID SOLUTION FROM SAID ANODEINTERIOR SPACE AND RECIRCULATING IT TO SAID TANK DURINGELECTRODEPOSITION OF SAID WEAR-RESISTAND COATING ONTO SAID ALIGNED INNERSURFACES.
 2. An apparatus for uniformly electrodepositing a chromiumcoating onto epitrochoidal radial working surfaces of a plurality ofrotary engine housings simultaneously, said apparatus comprising: anonconductive closed box-like enclosure for containing a vertical stackof abutting and registered rotary engine housings having alignedepitrochoidal inner working surfaces forming a passageway having anepitrochoidal cross section with a major dimension of about 1 - 12inches, flat metal plates about 0.1 - 0.2 inch thick contacting the topand bottom of said housing stack, with each plate having anepitrochoidal opening therein congruent with said aligned innersurfaces, flat top and bottom walls on the interior of said enclosureadjacent said flat metal plates with each wall having an epitrochoidalopening therein concentric with and having a major dimension of about1 - 1.5 inch smaller than said passageway major dimension, an insolubleanode having a generally epitrochoidal transverse outer periphery and aninterior space, said anode coaxially extending throughout saidpassageway parallel to and radially spaced inwardly from said alignedinner surfaces, said anode transverse outer periphery having a majordimension about 1.5 - 2.0 inches smaller than said passageway majordimension, said major dimension of said concentric epitrochoidalopenings in said enclosure top and bottom walls being larger than saidanode major dimension by about 2/3 of the difference between said anodeand said passage major dimensions, a tank having a bottom wall andcontaining a solution for electrodeposition of chromium, means forsupporting said enclosure in said tank completely immersed in saidsolution and spaced above said tank bottom wall wherein said solutioncan enter said passageway through said enclosure top and bottomopenings, said anode having a plurality of openings radially arrayedthroughout said passageway by which said solution can enter said anodeinterior space from said passageway, means for preventing said solutionfrom entering said anode interior space except through said anodeopenings in said paSsageway, and means for continuously withdrawing saidsolution from said anode interior space and recirculating it to saidtank during electrodeposition of said chromium coating onto said alignedinner surfaces of said housings.
 3. An apparatus for uniformlyelectrodepositing a chromium coating onto epitrochoidal radial workingsurfaces of a plurality of rotary engine housings simultaneously, saidapparatus comprising: a frame for supporting workpieces to beelectroplated in a tank containing chromium electroplating solution, ahorizontal first flat nonconductive plate on said frame, a verticallyoriented tubular nonconductive member with open upper and lower ends,the lower end of which is sealed on and encloses an upper surfaceportion of said first nonconductive plate, a first horizontal flat metalplate on said enclosed upper surface portion within said tubular memberfor contact with the bottom of a vertical stack of abutting andregistered rotary engine housings within said tubular member, saidhousings having aligned epitrochoidal inner working surfaces forming avertical passageway of epitrochoidal horizontal cross section, a secondhorizontal flat metal plate within said tubular member for contact withthe top of said stack of rotary engine housings, a second horizontalflat nonconductive plate on top of said second metal plate, andsubstantially closing the upper end of said tubular member, said firstand second flat metal plates each having an epitrochoidal openingtherein congruent with said aligned inner surfaces of said rotary enginehousings, said first and second flat nonconductive plates each having anepitrochoidal opening therein concentric with and smaller than saidmetal plate openings through which said electroplating solution in saidtank can enter top and bottom ends of said passageway, a verticaltubular insoluble anode having an epitrochoidal horizontal outerperiphery concentric with and smaller than said nonconductive plateopenings coaxially disposed in said housing passageway and uniformlyradially spaced inwardly from said aligned inner surfaces, the radialspacing between said anode and said passageway being about two to threetimes greater than the radial spacing from said nonconductive plates,said anode extending upwardly along the entire length of said passagewayand through said second plate opening to a level above them, anonconductive coating covering all anode portions outside saidpassageway and anode waist portions within said passageway, and saidanode having a closed bottom end and a plurality of uniformly radiallyarrayed openings therein along its length through which electroplatingsolution from said passageway can pass within said tubular anode and berecirculated to said tank during electroplating.
 4. A method ofuniformly electrodepositing a wear-resistant coating onto epitrochoidalworking surfaces of a plurality of rotary engine housingssimultaneously, said method comprising the steps of: vertically stackinga plurality of rotary engine housings having epitrochoidal inner workingsurfaces in alignment to form a passageway having an epitrochoidal crosssection, stacking metal plates at the top and bottom of the housingstack with the metal plates each having an epitrochoidal opening thereincongruent with said passageway, substantially enclosing said stack ofhousings and metal plates in a nonconductive container having top andbottom epitrochoidal openings therein adjacent the stack that areconcentric with and smaller than the epitrochoidal cross section of saidpassageway, coaxially positioning a tubular insoluble anode having ahollow interior and a generally epitrochoidal cross section within saidpassageway, with said anode paralleling the entire length of saidpassageway and spaced radially inwardly therefrom about two or threetimes its radial spacing from said container openings, said anode havingradially arrayed opeNings throughout said passageway communicating saidpassageway with the anode interior, completely immersing said containerand said anode in an electroplating bath so that electroplating solutioncan enter said container through said top and bottom openings,preventing electroplating solution from entering the anode interiorexcept through said radially arrayed openings, applying a negativepotential to said housing stack and a positive potential to said anode,and while applying said potentials, continuously withdrawing saidelectroplating solution from said anode interior and recirculating itback to said tank.